Fixed operators’ crucial role in mobile backhaul

Mobile evolutions are increasingly influenced by the availability of cost effective and geographically widespread backhaul solutions. Fixed networks are ideally positioned to address this need: while operators are expanding the fiber-to-the-home (FTTH) footprint to connect more residential and enterprise customers, they can also support more mobile broadband traffic, eliminating the need to create parallel and dedicated networks for backhaul.

Increasing synergies between mobile and fixed networks

A few years ago, many predicted that the rapidly increasing demand for mobile services would reduce the importance of fixed networks, if not render them obsolete. However, the opposite is becoming true. We have reached a point where mobile operators cannot efficiently meet rising demand without relying on fixed broadband networks. Fixed networks have become a strategic differentiator for mobile services and owner-operators of each type of network can benefit from closer collaboration.

Mobile and fixed operators are facing a very complementary set of challenges and opportunities. Both are trying to keep pace with increasing demand; more subscribers, generating more traffic and wanting faster connections. On the fixed front, fiber is being brought closer and closer to end-users. The resulting increase in street cabinets and nodes perfectly matches the cell densification strategy being pursued by mobile operators. While mobile operators have traditionally relied on dedicated networks to connect their cell sites to their core networks, they now have the opportunity to reduce costs by handing off mobile traffic to the fixed network.

Efficiency savings by converging networks

The mobile backhaul network must be able to scale to support increasing numbers of cell sites at higher capacities. Fixed broadband access networks are ideal due to their dense geographical presence in those areas where wireless congestion and expansion will occur. In fact, mobile operators are now positioning radio sites depending on the proximity of FTTx access points as this significantly lowers the cost for both site access and backhauling.

This leads to an obvious question: why shouldn’t every fixed network node support a mobile wireless node? In essence, we can consider mobile networks as fixed networks with wireless tails where the fixed network is used for mobile backhaul. Converging fixed and mobile services on a single access platform provides greater operational flexibility, simpler network design and opens new markets for operators..

As one of our North American customers puts it: “Because the investment [in a fixed network] was already been made, we might as well leverage that investment and hang more services on it."[1] At the same time, this operator continues to add fiber to more towers and fiber-based backhaul overall continues to drive revenues for their wholesale division.

Network boundaries are blurring

Since mobile services took off in the 1980s, to a large extent they have developed separately from fixed services. Even where one operator owned both fixed and wireless networks, the assets were managed by separate business divisions or even separate companies. Incumbent telecoms operators and new mobile players deployed dedicated networks for mobile services because they had separate budgets, separate operations and different service level requirements.

But over the last decade or so, mobile and fixed operators across the globe have been converging through mergers and acquisitions as they have begun to recognize the complementarity of fixed and mobile networks. Regulators will often enforce non-discriminatory network access between infrastructure and service providers to ensure a competitive market and profitable business for operators. This opens up opportunities for all players: wholesale providers, MVNOs, converged operators and mobile-only operators. The boundaries are blurring and fixed and mobile networks are now more complementary than they ever were competitive. Operators who succeed in making greater use of fixed networks will improve the return on their investments and gain significant cost advantages.

Fiber for the future of backhaul

Historically, mobile backhaul capacity would be leased from a network operator in the form of E1/T1 leased lines. The cost of these wired solutions was typically prohibitive unless infrastructure was readily accessible, i.e. close to cell towers. Today, thanks to high demand for high-speed internet access, there is a considerable likelihood of a fixed access node being available in most areas. The incremental cost of backhauling mobile traffic on top of today’s FTTx networks is far less than the cost of building out a separate network.

Backhauling increasingly uses fiber-based technologies. Similar to the drive for FTTH, an ideal scenario is to deploy fiber to every radio cell site location: fiber-to-the-cell (FTTCell). This can be achieved with point-to-point (P2P) fiber connections or a point-to-multipoint (P2MP) Passive Optical Network (PON) technology, which greatly reduces the cost of deploying and maintaining the fiber network. Today’s PON technologies such as GPON (Gigabit PON) can accommodate foreseeable mobile backhaul bandwidth needs and higher bandwidth PON evolutions such as XGS-PON (up to 10 Gb/s) and TWDM-PON (up to 40 Gb/s via multiple wavelengths) are already available. These technologies can be gradually introduced into the network as the need for more bandwidth grows, ensuring a future-ready investment for operators.

Leveraging existing FTTX networks

However, some locations will always be hard to reach with fiber. Trenching cable, digging up pavements, accessing distribution points in the streets and multi-dwelling units adds significant complexity and cost. In these cases, the extensive installed base of existing copper plant can also be used for mobile backhaul. Today’s digital subscriber line (DSL) technologies provide plenty of bandwidth for both fixed and mobile traffic (e.g. up to 100 Mb/s with VDSL2 vectoring). Bandwidth can be further augmented by bonding (using two or more copper twisted pairs together to multiply capacity) or with new technologies such as Vplus, G.fast and XG-FAST, which provide fiber-like speeds (up to multiple gigabits per second). An existing copper access network can often be the most cost-effective and the fastest time-to-market option for mobile backhaul.

Backhaul-ready infrastructure

Whether copper or fiber, there are three critical requirements for a mobile backhaul network: resiliency, quality of service (QoS) and time synchronization.

The demands placed on fixed broadband networks for video, voice and data are very similar to those needed for mobile backhaul. While pursuing residential broadband, fixed operators have actually been deploying a backhaul-ready infrastructure with prime QoS and multiservice transport capabilities.

The mobile backhaul network must support network synchronization techniques where access to external sources is unavailable or prohibitively expensive. Synchronization is even more relevant for LTE-Advanced, which requires progressively more accurate phase (and time of day) information in order to exploit spectral efficiencies to the maximum. Fixed networks support built-in architectural features, efficiently tuned algorithms and powerful QoS mechanisms to minimize the latency experienced by synchronization traffic.

Network sharing strategies

Service providers of every kind are increasing investments to expand their reach and improve speeds for customers. The monetization of their assets and return on investment are, to some extent, even more important than choosing which technology to use. Operators can consider 3 different levels of sharing fixed network assets for mobile backhaul: spatial, spectral and temporal.

Spatial sharing simply means running dedicated, parallel infrastructure for each service but sharing common outside plant like ducts and poles and potentially even cables, though maintaining separate fiber lines for each service. This option nicely segregates operations at the expense of duplicating active equipment. Operators can also share the same access platform but not the same cables by having a dedicated PON for backhaul services. In this case, reduced split ratios (1:4 or 1:8) can be used for backhaul, such that the PON bandwidth is shared between fewer connection points.

Spectral sharing means using the same physical fiber for all services but giving each service a separate, dedicated wavelength. This technique can use different P2P WDM technologies such as Coarse Wave Division Multiplexing (CWDM) and WDM-PON for each service. However, P2P and P2MP technologies cannot be combined without investing in separate fibers or additional wavelength multiplexers to separate traffic. This is a complex and expensive solution that restricts future scalability of the network.

The most cost effective solution is to share both the access equipment and the fibers of the OSP. This can be reached via temporal sharing: using the same transportation technology on the same fiber with services allocated to different timeslots. Timeslots can be fixed for fully segregated services, but the most cost effective technique is dynamic allocation. Services can be dynamically mixed based on a statistical usage or oversubscription of the medium.

There are two ways to do this:

Multiplex Ethernet packets of different services on the same PON. Even on heavily loaded GPONs that are popular for residential FTTH services, there will generally be more than 1 Gb/s headroom left for adding one or two cell sites. This is particularly effective, and easy to achieve, for urban and sub-urban cell sites (e.g. rooftops of offices or multi dwelling units). Statistical multiplexing manages the Ethernet packets between FTTH and FTTCell: FTTH traffic mainly peaks during the morning and evening while cell traffic mainly peaks during the day.

Deploy multiple wavelengths with TWDM-PON. TWDM-PON adds additional P2MP wavelengths on the same PON infrastructure. Bandwidths can be allocated flexibly for each wavelength and each can evolve independently. Wavelengths can also be unbundled and assigned to additional operators, which promotes co-investment opportunities that allow operators to effectively share a common infrastructure but manage, control, and operate their own dedicated wavelength. This significantly lowers the costs and risks of deploying a fiber network and creates an easy way to increase the addressable market, with the same investment.

About Filip de Greve

As product marketing manager at Nokia, Filip focuses on new market strategies for innovative copper and fiber based access solutions. He holds a Ph.D. in telecomm and has over 10 years experience in the ICT & telecom industry with broad expertise in hardware, software and services. He loves road cycling and thrives on thinking outside the box to explore new ideas and concepts.